Current-limiting switching arrangements are designed in particular to interrupt current paths in the event of a short-circuit or an overcurrent. Furthermore, current-limiting switching arrangements can be designed to be single-pole or multi-pole, in particular three-pole. They can have one or more pairs of switching contacts per arc contact. In particular these electrical switching arrangements are designed to interrupt currents of more than 100 A, in particular of several kA.
Thus for example when using current-limiting switching devices, in particular current-limiting circuit-breakers for example in the form of MCCBs (Molded Case Circuit Breaker) in widely branched power distribution grids, selective gradings with a minimum rated current spacing of the relevant switching devices is normal. Each branching level can in this case be protected, as a function of the consumers connected, against overloads and short-circuits that occur, using a correspondingly dimensioned switching device.
In this case, for example, a switching device which is arranged closest to a consumer and which is often also referred to as a switching device which is close to or downstream of the consumer, is designed for the lowest rated current. If a short-circuit current now flows both through the switching device close to the consumer and also through a switching device that is arranged in the hierarchy of the power distribution grid above the device close to the consumer and is often also referred to as a switching device which is remote from or upstream of the consumer, only the switching device close to the consumer should now disconnect. In other words in the event of a fault (short-circuit) only the switching device which is closest to the event should interrupt the flow of current.
The pairs of switching contacts of the switching device which is close to and sometimes also of the switching device which is remote from the consumer draw an arc when opened, the width of opening of the pairs of switching contacts and also the arc power in the case of the switching device close to the consumer being higher because of the lower moment of inertia of its moving current path including the switching contacts. This opening, which is sometimes only single-pole, must be followed by an all-pole disconnection of the switching device close to the consumer. The switching device remote from the consumer must not disconnect, in order not to disconnect other consumers from the power distribution grid. The switching device remote from the consumer may however act in a supporting capacity by briefly disengaging the switching contacts, thus for example helping to disconnect the switching device close to the consumer by limiting the current.
Switching devices which work in this graded manner in power distribution grids behave selectively. To achieve this selectivity it is necessary that the switching devices located closest to the fault interrupt the current paths of all arc contacts and that the higher-level switching devices remain on the grid.
Generic releases and switching arrangements with releases of this type which are suitable for such selective interruption of current paths are known for example from DE 10 2009 015126 A1.
DE 10 2009 015126 A1 discloses a release for an electrical switching arrangement which is arranged over the course of a first current path and which has at least two switching contacts arranged in a housing which are isolated if the current flowing across the switching contacts has exceeded a particular threshold value, with an actuating element, which counter to the force of a restraining apparatus responds to a pressure which is generated by an arc drawn in the event of an electrodynamic recoil of the switching contacts in an isolating zone of the switching contacts surrounded by the housing, and which actuates a shutdown mechanism effecting the automatic interruption of the current path, the actuating element having a movable element which forms a blocking element in a flow channel connected to the isolating zone, which blocking element performs a predetermined control movement at the pressure which is intended to result in disconnection.
Generally in the case of selective releases it is necessary to ensure that they continue to work even after the contacts have opened several times. When contacts open this can lead to the formation of carbon black, metal condensate, metal beads and burn-off products of plastics. These substances can be deposited on the movable element of a selective release or in the vicinity thereof and hence impede its working.
The moving element of a selective release according to DE 10 2009 015126 A1 can be mounted in only one swivel axis, which moreover can be designed with clearance. As a result, if the contacts are opened by the pressure of the arcing gases it can happen that the moving element is likewise pressed in the direction of the flow and comes into contact with parts of the housing. This contact between a moving element and parts of the housing and the consequent friction between them can result in a failure or in an incorrect release of the selective release. The friction between the moving element and the parts of the housing can additionally be increased by contamination which arises as a result of the opening of the contacts.